AngularFireDatabase get list names

Hey everyone. I had a Firebase Database see with structure (see Screenshot):


I want to output ( horro, action and Musikfilme) - how can I do it? my code is this:

this.fdb.list('/kategorien/').valueChanges().subscribe( data => {
      //Todo: output name not key 
      this.arrMedia =  Object.keys(data); 
      console.log("Medien: " + this.arrMedia);

But I become only key ( 0, 1, 2) anyone have ideas?